In recent years, there has been known a technique for detecting a lane mark such as a white line on a road by capturing an image of a road along which a vehicle travels and processing the acquired image with an imaging means such as a CCD camera mounted on the vehicle and for controlling the vehicle or providing a driver with information on the basis of information on a lane (traffic lane) along which the vehicle travels recognized from a detection result (refer to, for example, Japanese Patent Application Laid-Open No. Hei 11-147473/1999 (hereinafter, referred to as Patent Document 1).
The steering force assisting device in Patent Document 1 extracts a point where luminance changes from dark to light (positive edge point) and a point where luminance changes from light to dark (negative edge point) on the basis of a peak of a derivative value with the luminance value on each horizontal line differentiated from the left in the lateral direction regarding a plurality of horizontal lines on the road image. Then, a combination of edge points, in which the positive edge point and the negative edge point appear in this order on each horizontal line and in which the edge points are arranged at intervals considered to be appropriate for a white line, is extracted as a white line candidate. Then, a white line is detected from the extracted white line candidates on the basis of its position on the image.
However, there may be, for example, a trail (track) of wheels of the vehicle traveling along the road besides a lane mark such as a white line on the road. The wheel track often appears along the white line in the vicinity thereof and changes in luminance in the edge portion like the white line. Therefore, the wheel track has similar features to the white line on the image. Particularly, on the wet road like a road after rain, the edge portion of the wheel track reflects light, by which a luminance difference between the edge portion and the road surface is similar to a luminance difference between the white line and the road surface. Consequently, it is difficult to distinguish the lane mark such as a white line from a wheel track or the like other than the lane mark on the road using the technique described in Patent Document 1, thereby incorrectly detecting the object other than the lane mark as a lane mark disadvantageously.